Our invention relates to dielectric ceramic compositions that allow sintering or firing at lower temperatures than heretofore, and to a process for the fabrication of coherently bonded bodies of such compositions. The low temperature sintered ceramic materials of our invention find a typical application in solid dielectric capacitors, particularly in those of the monolithic or multilayered design, making possible the use of a base metal as the material for the film electrodes of such capacitors.
Multilayered ceramic capacitors are known which employ noble metals such as platinum and palladium as the electrode materials. For the manufacture of such multilayered capacitors, as heretofore practiced, there are first prepared green dielectric sheets from the proportioned ingredients for a dielectric ceramic material in finely divided form. An electroconductive paste containing powdered platinum or palladium is then "printed" on the green sheets in a desired pattern. Several such printed green sheets are stacked up, pressed together, and sintered in an temperature range of 1300.degree. to 1600.degree. C. In an oxidative atmosphere. This conventional method makes possible the simultaneous production (co-sintering) of the dielectric ceramic bodies and the film electrodes interleaved therein. It is also an acknowledged advantage of the known method that the noble metal electrodes are totally unaffected by the high temperature sintering in an oxidative atmosphere. Offsetting all these advantages is the expensiveness of the noble metals, which adds considerably to the costs of the multilayered ceramic capacitors.
A solution to this problem is found in Sakabe et al. U.S. Pat. No. 4,115,493 issued Sept. 19, 1978. This patent proposes a method of making a monolithic ceramic capacitor with use of a base metal such as nickel, iron or cobalt, or an alloy thereof, as the electrode material. According to this prior art method, a paste of a powdered base metal is screened on green dielectric sheets, and the pasted sheets are stacked, pressed together, and fired in a temperature range of 1300.degree. to 1370.degree. C. in a reductive atmosphere containing hydrogen. The oxidation of the pasted base metal particles is thus avoided.
We do, however, object to the Sakabe et al. method for the following reasons. We have discovered from experiment that the firing temperature range of 1300.degree. to 1370.degree. C. suggested by this prior art method leads to easy flocculation of the particles of nickel or like base metal pasted on the green dielectric sheets. The base metal particles have also been found to tend to diffuse into the ceramic bodies when fired in that temperature range. The flocculation and diffusion of the base metal particles are, of course, both undesirable as the resulting capacitors will not possess desired values of capacitance and insulation resistance.